The present invention is widely applicable to the production in general of a material for semiconductors or electronic devices such as semiconductor device and liquid crystal device, but for the sake of convenience, the background art of semiconductor devices is described here as an example.
With recent miniaturization of semiconductor devices, needs for a thin and good-quality silicon oxide film (SiO2 film) are significantly increasing. For example, in an MOS-type transistor structure (FIG. 1) which is a most popular constitution of semiconductor devices, a very thin (for example, about 2.5 nm or less) and good-quality gate insulating film (SiO2 film) according to the so-called scaling rule is keenly demanded.
The gate insulating film material heretofore used in industry is a silicon oxide film (SiO2 film) obtained by directly oxidizing a silicon substrate in a high-temperature heating furnace of approximately from 850 to 1,000° C.
However, if such an SiO2 film is merely made thin to 2.5 nm or less, the leak current passing through the gate insulating film (gate leak current) becomes large and this causes problems such as increase of power consumption or accelerated deterioration of device properties.
Furthermore, in using a conventional thin gate insulating film, boron contained in a gate electrode (mainly polysilicon) of a P-type MOS transistor penetrates into the SiO2 film at the formation of the gate electrode and the boron concentration of the gate electrode changes to cause a problem that the semiconductor device properties deteriorate. As one of the methods for solving such problems, studies are being made to use an oxynitride film (acid nitride film) as the gate insulating film material.
When nitrogen is contained in the insulating film, this is advantageous in that the dielectric constant of the film is elevated and the electrical capacitance (capacitance) increases as compared with an oxide film having the same physical film thickness. The MOS-type transistor, which can be typically represented by the structure shown in FIG. 1 described later, contains an MOS (metal-oxide-semiconductor) capacitor structure using a gate insulating film as the dielectric material, between two metals (doped polysilicon (gate electrode) and silicon substrate).
In order to attain high-speed transistor operation, the time required for a carrier to move between the source and the drain shown in FIG. 1 must be shortened. The measure therefor includes two approaches, that is, a method of increasing the speed (mobility) of the carrier moving between the source and the drain, and a method of reducing the distance between the source and the drain. At present, the control of the interface between the silicon substrate and the oxide film reaches the limit and the mobility can be hardly increased any more.
Accordingly, the method of reducing the channel length in the MOS structure of FIG. 1 is used at present for achieving high-speed operation of transistor. As this channel length is shorter, the time required for a carrier to move becomes shorter and a transistor operation at a higher speed can be realized. However, the reduction of the channel length has the same meaning as the reduction in the area of MOS capacitor contained in that portion, namely, reduction in the capacitance, and this gives rise to an insufficient amount of carrier (electron or hole) induced at the operation and in turn, difficulty in obtaining an S/N ratio high enough to bring about the operation. Accordingly, in order to realize a device having high-speed operation reliability, a measure must be taken to maintain the capacitance even when the area is decreased.
As the measure therefor, a method of decreasing the film thickness of the gate insulating film of FIG. 1 has been conventionally employed, but the thinning of the film incurs the following problems. One problem is that a current (leak current) flows between the silicon substrate (channel) and the gate electrode due to quantum mechanical tunnel effect and the power consumption increases. A low power consumption device is essential for the development of portable electronic devices in a recently started ubiquitous society (information society allowing for connection to the network at any time and any place through an electronic device as the medium), and reduction of this leak current is an important problem.
Also, with the thinning of the gate oxide film, as described above, the penetration of boron from the gate electrode of a P-type MOS transistor comes out as a serious problem. Boron has a property of readily passing through an oxide film and as the film is more thinned, this causes a problem that boron penetrates the oxide film and the boron concentration (dope amount) of the gate electrode changes. A CMOS structure (mixed-mounting of N-type and P-type transistors) is fundamental for low power consumption devices and accordingly, the presence of a P-type MOS transistor is indispensable. The change in the dope amount of the gate electrode causes change in the threshold voltage of the transistor and the transistor may undergo irregular operation. Therefore, it is very important to prevent the penetration of boron.
In order to solve these problems, as described above, a method of incorporating nitrogen into the silicon oxide film has been proposed. When nitrogen contained, this is known to elevate the dielectric constant and prevent the penetration of boron.
However, if such an oxynitride film is directly and simply formed by a thermal oxynitridation method, nitrogen is contained in a large amount at the interface with the silicon substrate and the device properties inevitably tend to be deteriorated. When nitrogen is contained at the interface, this is known to cause deterioration in the mobility of carrier and in turn in the transistor operation properties. Also, in an SiO2/SiN stack structure combining the formation of thermal oxide film and the formation of SiN film by CVD (chemical vapor deposition), a trap (in-film level) of carrier is generated at the SiO2/SiN interface and this tends to cause deterioration of the device properties, such as shifting of the threshold voltage.
When the SiO2 film is intended to nitride by heating, a high temperature of 1,000° C. or more is usually necessary and in this heat step, differential diffusion of the dopant injected into the silicon substrate is liable to occur to deteriorate the device properties (such a method is disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 55-134937 and 59-4059).